There is known a background image forming apparatus which forms a toner image on a surface of a drum-shaped photoconductor through a well-known electrophotographic process.
The structural configuration of such an apparatus is as follows. An endless intermediate transfer belt is brought into contact with the photoconductor to form a primary transfer nip. In the primary transfer nip, the toner image on the photoconductor is primarily transferred onto the intermediate transfer belt. A secondary transfer roller is brought into contact with the intermediate transfer belt to form a secondary transfer nip. In the loop of the intermediate transfer belt, a secondary transfer opposite roller is disposed. The intermediate transfer belt is nipped between the secondary transfer opposite roller and the above-described secondary transfer roller. The secondary transfer opposite roller disposed inside the loop is grounded. By contrast, a secondary transfer bias is applied to the secondary transfer roller disposed outside the loop. Between the secondary transfer opposite roller and the secondary transfer roller, therefore, a secondary transfer electric field is generated which electrostatically moves the toner image from the side of the secondary transfer opposite roller toward the side of the secondary transfer roller. With the action of the secondary transfer electric field and nip pressure, the toner image on the intermediate transfer belt is secondarily transferred onto a recording sheet conveyed into the secondary transfer nip in synchronization with the toner image on the intermediate transfer belt.
In the above-described configuration, with recording media with substantial surface roughness, such as a Japanese paper sheet, an uneven toner image density pattern conforming to the surface roughness tends to be formed in the toner image, owing to a failure to transfer a sufficient amount of toner to recesses in the surface of the sheet.
Accordingly, the background image forming apparatus employs, as the secondary transfer bias, a superimposed bias including an alternating current (AC) voltage component superimposed on a direct current (DC) voltage component, instead of a bias including only a DC voltage. It has been shown experimentally that it is possible to minimize the formation of an uneven density pattern with the application of the above-described secondary transfer bias, as compared with the application of the secondary transfer bias including only the DC voltage.
However, the present inventors have found from experiments that there are cases in which a sufficient image density fails to be obtained in the recesses in a sheet surface, even with application of a superimposed bias as the secondary transfer bias. It was also found that, even if a sufficient image density is successfully obtained in the recesses, a plurality of white spots appear in an image area corresponding to the recesses.
Upon closer inspection, the present inventors found the following phenomenon, described below with reference to FIGS. 1 and 2.
FIG. 1 is an enlarged configuration diagram of a related art image forming apparatus 530 illustrating an example of the secondary transfer nip. In the drawing, an intermediate transfer belt 531 is pressed against a nip formation roller 536 by a secondary transfer inner surface roller 533 in contact with the inner surface of the intermediate transfer belt 531. With this pressing, a secondary transfer nip is formed in which the outer surface of the intermediate transfer belt 531 and the nip formation roller 536 come into contact with each other. A toner image on the intermediate transfer belt 531 is secondarily transferred onto a recording sheet P conveyed into the secondary transfer nip. The secondary transfer bias for secondarily transferring the toner image is applied to one of the two rollers illustrated in the drawing, and the other roller is electrically grounded. It is possible to transfer the toner image onto the recording sheet P, irrespective of to which of the rollers the transfer bias is applied.
Herein, a description is given of a case of applying the secondary transfer bias to the secondary transfer inner surface roller 533 and using toner of negative polarity. In this case, to move the toner in the secondary transfer nip from the side of the secondary transfer inner surface roller 533 toward the side of the nip formation roller 536, a bias having a time-averaged electrical potential of the same negative polarity as the polarity of the toner is applied as the secondary transfer bias including a superimposed bias.
FIG. 2 is a waveform chart illustrating an example of a waveform of the secondary transfer bias including a superimposed bias and applied to the secondary transfer inner surface roller 533. In the drawing, an offset voltage Voff in volts (V) represents the time-averaged value of the secondary transfer bias. As illustrated in the drawing, the secondary transfer bias including a superimposed bias has a sinusoidal waveform, and includes a positive peak value and a negative peak value. A reference sign Vt represents one of the two peak values for moving the toner in the secondary transfer nip from the belt side toward the recording sheet side, i.e., the negative peak value in the present example (hereinafter referred to as the transferring peak value Vt). A reference sign Vr represents the other peak value for returning the toner from the recording sheet side toward the belt side, i.e., the positive peak value in the present example (hereinafter referred to as the returning peak value Vr). Vpp represents the peak-to-peak voltage.
Even if an AC bias including only an AC component is applied instead of the superimposed bias as illustrated in the drawing, it is possible to move the toner back and forth between the intermediate transfer belt 531 and the recording sheet P in the secondary transfer nip. The AC bias, however, simply moves the toner back and forth, and is unable to transfer the toner onto the recording sheet P. If a superimposed bias including a DC component is applied to adjust the offset voltage Voff, i.e., the time-averaged value of the superimposed bias, to the same negative polarity as the polarity of the toner, it is possible to cause the toner to relatively move from the belt toward the recording sheet P during the back-and-forth movement thereof, and thereby to transfer the toner onto the recording sheet P.
The present inventors have observed the behavior of the toner in the secondary transfer nip in the above-described configuration, and found that, when the secondary transfer bias including a superimposed bias starts being applied, only a very small number of toner particles present on the surface of a toner layer on the intermediate transfer belt 531 first separates from the toner layer and moves toward recesses in the surface of the recording sheet P. Most of the toner particles present in the toner layer remain therein. The very small number of toner particles having separated from the toner layer enters the recesses in the surface of the recording sheet P. Thereafter, if the direction of the electric field is reversed, the toner particles return from the recesses to the toner layer. In this process, the returning toner particles collide with the other toner particles remaining in the toner layer, and reduce the adhesion of the other toner particles to the toner layer. Then, in the next reversal of the direction of the electric field to the direction for moving toner particles toward the recording sheet P, a larger number of toner particles than in the first cycle separates from the toner layer and moves toward the recesses in the surface of the recording sheet P. As the above-described sequence is repeated, the number of toner particles separating from the toner layer and entering the recesses in the surface of the recording sheet P is gradually increased. Consequently, a sufficient amount of toner particles is eventually transferred into the recesses.
However, it was found that, if the toner adhesion amount in the toner layer is relatively large, it is difficult for the returning peak value Vr illustrated in FIG. 2 to cause the toner particles transferred into the recesses in the surface of the recording sheet P to return to the toner layer on the intermediate transfer belt 531, and that this difficulty results in a deficiency in image density in the recesses. It was also found that, if the toner adhesion amount in the toner layer is relatively small, white spots tend to appear in the image in the area of the recesses in the surface of the recording sheet P, when the secondary transfer bias reaches the transferring peak value Vt. For example, the potential difference between the secondary transfer inner surface roller 533 and the nip formation roller 536 illustrated in FIG. 1 reaches its maximum when the secondary transfer bias reaches the transferring peak value Vt. In this state, discharge tends to occur from the side of the secondary transfer inner surface roller 533 toward the side of the nip formation roller 536 in the recesses in the surface of the recording sheet P. In this case, if the toner adhesion amount in the toner layer is relatively large, toner particles having a polarity that is the opposite of the polarity of the transferring peak value Vt are present between the secondary transfer inner surface roller 533 and the recording sheet P. Therefore, the above-described discharge is minimized. Meanwhile, if the toner adhesion amount in the toner layer is relatively small, there are fewer toner particles opposite in polarity to the transferring peak value Vt and present between the secondary transfer inner surface roller 533 and the recording sheet P. Therefore, the above-described discharge occurs. As a result, the toner particles oppositely charged by the discharge are hardly transferred into the recesses in the surface of the recording sheet P, and a multitude of white spots appear in the image in the image area of the recesses in the surface of the recording medium.